Aging of α-Pinene Secondary Organic Aerosol by Hydroxyl Radicals in the Aqueous Phase: Kinetics and Products

The reaction of hydroxyl radicals (OH) with a water-soluble fraction of the α-pinene secondary organic aerosol (SOA) was investigated using liquid chromatography coupled with negative electrospray ionization mass spectrometry. The SOA was generated by the dark ozonolysis of α-pinene, extracted into the water, and subjected to chemical aging by the OH. Bimolecular reaction rate coefficients (kOH) for the oxidation of terpenoic acids by the OH were measured using the relative rate method. The unaged SOA was dominated by the cyclobutyl-ring-retaining compounds, primarily cis-pinonic, cis-pinic, and hydroxy-pinonic acids. Aqueous oxidation by the OH resulted in the removal of early-stage products and dimers, including well-known oligomers with MW = 358 and 368 Da. Furthermore, a 2- to 5-fold increase in the concentration of cyclobutyl-ring-opening products was observed, including terpenylic and diaterpenylic acids and diaterpenylic acid acetate as well as some of the newly identified OH aging markers. At the same time, results obtained from the kinetic box model showed a high degree of SOA fragmentation following the reaction with the OH, which indicates that non-radical reactions occurring during the evaporation of water likely contribute to the high yields of terpenoic aqSOAs reported previously. The estimated atmospheric lifetimes showed that in clouds, terpenoic acids react with the OH exclusively in the aqueous phase. Aqueous OH aging of the α-pinene SOA results in a 10% increase of the average O/C ratio and a 3-fold decrease in the average kOH value, which is likely to affect the cloud condensation nuclei activity of the aqSOA formed after the evaporation of water.


INTRODUCTION
Secondary organic aerosols (SOAs) are formed from isoprene and terpenes emitted primarily from biogenic sources. 1 The oxidation of these atmospherically abundant, volatile organic compounds 2,3 forms particulate matter (PM), which affects the climate and visibility and negatively impacts human health. 4,5 Carboxylic acids are ubiquitous in the troposphere. 6,7 C 1 −C 4 acids contribute to the acidity of atmospheric aqueous particles, and their presence in cloud water often signifies the aqueous chemical aging of organic aerosols. 6−9 Gas-phase oxidation of the most abundant monoterpene, α-pinene (α-P), 1 generates large amounts of functionalized carboxylic acids, in addition to other oxygenated products. 10−13 Multifunctional terpenoic acids (TAs) and acidic dimers contribute significantly to monoterpenoic SOAs; 14−18 these compounds also play an important role in the SOA particle formation and growth. 16,19,20 Furthermore, some poly(carboxylic acid)s, including 3-methyl-1,2,3-butanetricarboxylic (MBTCA) and hydroxydicarboxylic acids, were identified as important aging markers of terpenoic SOAs. 21−24 Due to their high values of Henry's law constants (10 7 −10 13 M × atm −1 ), 25 TAs and acidic dimers reside almost entirely in cloud and fog water. 26−28 Therefore, such molecules are likely to undergo aqueous chemical aging, which can contribute to the formation of low-volatility organics in the atmosphere. 26,29−33 Aqueous photochemical processing alters the molecular composition and physicochemical as well as optical properties of SOAs. 33−37 Consequently, the formation and processing of SOAs in the aqueous phase have a direct impact on their cloud condensation nuclei and ice-nuclei activity, 38 particle mass and number concentrations as well as the health effects of atmospheric PM. 39−42 Because liquid water is a major tropospheric constituent, 39,43 formations of aqueous SOAs ( aq SOAs) are expected to significantly contribute to the global budget of organic aerosols. 44 At the same time, the formation and processing of aq SOAs 43,45 are still not well characterized, 43,46,47 which limits our understating of the climate forcing of organic aerosols.
Aqueous processing (aging) of terpenoic precursors by the OH, which is the major atmospheric oxidant during the daytime, 5,46 recently received attention as the potential source of aq SOAs. 26,29,33,48−54 The kinetic data available to date strongly indicate that OH oxidation of TAs is likely to occur under realistic atmospheric conditions. 49,50,52,53 Aqueous OH aging of the water-soluble fraction of α-P SOA (α-P SOAd aq ) yielded low-volatility, oxidized products 29,31,33 but was also shown to decompose dimers. 33,48 Atmospheric multiphase modeling of the formation and aging of SOAs requires detailed chemical mechanisms. 44,55,56 However, a large number of studies published to date have been focused on single precursors, 26,27,29,[31][32][33]57 which limits the current understanding of the mechanisms of aqueous photochemical aging of terpenoic SOAs.
In this work, the aqueous oxidation of α-P SOAd aq by the OH was investigated: This work presents a first attempt at an explicit parameterization of the aqueous processing of α-P SOA by the OH using a kinetic model. The goals of this work included investigating the potential of the α-P SOAd aq to undergo a reaction with the OH in atmospheric water-containing particles and studying the kinetics and products of R(I).
Here, an α-P SOA was generated in the flow-tube rector, 58 collected on a filter, extracted into the water, and oxidized by the OH in the aqueous photoreactor. 54 This work was focused on studying the chemical aging of the atmospherically abundant TAs and acidic oligomers contributing to α-P SOAd aq . 17,59,60 For this reason, aliquots of the reaction solution were analyzed with liquid chromatography coupled with negative electrospray ionization mass spectrometry (LC-ESI(−)/MS). 14,17,59,60 LC-ESI(−)/MS is capable of detecting as much as 72% of α-P SOAd aq . 14 With the use of this hyphenated technique, dimers are also unambiguously distinguished from the lower-MW products, thereby providing detailed insight into the formation and evolution of individual TAs following R(I).
The bimolecular reaction rate coefficients for the oxidation of individual water soluble organic compounds (WSOCs) contributing to α-P SOA by the OH (k OH , M −1 s −1 ) were measured using the relative rate method. 51 The WSOCs under investigation were also analyzed with LC coupled with highresolution MS to elucidate the structures of the detected tracers of α-P SOA .

EXPERIMENTAL SECTION
Materials and reagents are listed in Supporting Information (SI) Section S1. In all experiments, deionized water (18 MΩ × cm −1 ) was used.
2.1. Generation of α-Pinene SOA. Approximately 10 ppm of α-P and O 3 were reacted in a 17 L flow-tube reactor under laminar flow conditions (5 min residence time). 58 Relative humidity was 5%, and no OH scavenger was used. The SOA was collected on a 47 mm filter (EMFAB TX40H120-WW, Pall) for 2 h and kept at −85°C before the aging experiments. Filters were extracted via mechanical agitation with 3 mL of ACN/H 2 O (1:1, v/v) for LC-ToF/MS measurements (Section 2.3) or with 70 mL of water for aqueous OH aging (Section 2.2) and kinetic measurements (Section 2.4).

Aqueous Oxidation by the OH.
The solution of α-P SOAd aq was transferred into a jacketed, Pyrex reaction flask with an internal volume of 60 mL; all experiments were carried out at 298 K in the aqueous photoreactor. 54 The OH precursor (H 2 O 2 , concentration 10 mM) was photolyzed with four 9 W UVB lamps (PL-S 9 W/01/2P, Philips, peak emissions 310 nm). The reaction time was 60−80 min. The steady-state concentration of OH in the aqueous phase ([OH] aq ) in the absence of organic reactants was approximately 6 × 10 −13 M, as estimated by fitting the measured rate of H 2 O 2 photolysis into a kinetic model. 61,62 This value is within the range of [OH] aq in atmospheric aqueous particles. 63,64 2.3. Identification of α-Pinene Oxidation Products with Liquid Chromatography Coupled with High-Resolution Mass Spectrometry. The composition of the unaged SOA was studied with an LC20A liquid chromatograph coupled with an IT-TOF mass spectrometer (Shimadzu). The Waters Symmetry C18 column (150 mm × 2.1 mm, 3.5 μm, 100 Å), an eluent flow rate of 0.2 mL/min, ACN, and an aqueous solution of formic acid (0.03% v/v, pH = 2.8) were used. The gradient program was as follows: 0−2 min 5%B, 2− 3 min increase to 12%B, 3−11 min 12%B, 11−12 min increase to 18%B, 12−28 min 18%B, 28−41 min increase to 95%B, 41−45 min 95%B, 45−46 increase to 5%B, and the total analysis time was 56 min. The ESI ion source was operating in the negative ionization mode, the spray voltage was −3.5 kV, the nebulizing gas flow was 1.5 mL/min, and the source temperature and desolvation line temperature were 200°C. The detected [M-H] − ions were assumed to be singly charged, deprotonated pseudo-molecular ions containing C, H, and O atoms. O/C values and double-bond equivalents were calculated with the acquired HR/MS data.

Kinetic Measurements Using Liquid Chromatography Coupled to Triple Quadrupole Mass Spectrometry.
Aliquots of the reaction solution were periodically sampled from the photoreactor, quenched with catalase, 51 filtered through 0.22 μm PTFE syringe filters, and injected into the Nexera X2 liquid chromatograph coupled with the LCMS-8040 triple quadrupole mass spectrometer (Shimadzu) equipped with an ESI ion source. The LC conditions were the same as those described in Section 2.3. The mass spectrometer operated at unit mass resolution; interface and desolvation line temperatures were 250°C, and the heating block temperature was 350°C. The ESI voltage was −5 kV, and nitrogen was used as the source and collision gas. Concentrations of analytes were monitored in the multiple reaction monitoring modes (Table S1).
Kinetic reference compounds, suberic, camphoric, azelaic, and sebacic acid, 51 were added to the reaction solution before photooxidation. The pH of the reaction mixture was adjusted to 2 or 9 with HClO 4 or NaOH to measure the k OH values for the completely protonated and deprotonated TAs with pK a values between 4 and 5. 65 k OH values for the α-P SOAd aq constituents were derived using eq 1. i k j j j j j j j y In eq 1, subscripts refer to the initial (0) and intermediate  Compounds are listed in their retention order (Table S1) on the C18 HPLC column used, roughly corresponding to the order of decreasing polarity and increasing hydrophobicity. b For the compounds, tentatively identified in this work, substituent positions and only systematic names were given. c Structural isomers could not be distinguished based on the acquired MS data. d The value of the relative peak areas in the unaged SOA. e The reference is listed only if the structure was proposed, 14 based on the detailed interpretation of the acquired MS spectra. diffusion-controlled reactions (k diff ) were estimated with the Smoluchowski equation (Section S3). 66

Kinetic Box Model.
In the kinetic box model, 8hydroxy-pinonic, cis-pinonic, and cis-pinic acids were selected as the precursors for the rest of the TAs detected; pinonaldehyde and norpinonaldehyde, well-known products of α-P ozonolysis, were also included as precursors. 67,68 All reactions in the model are listed in Table S6, and the initial concentrations of TAs under investigation in the reaction solution were between 1 and 150 μM (Table S7), as estimated with the standard solution of cis-pinonic acid using chromatographic peak areas for the TAs under investigation. These concentrations are comparable with the amounts of dissolved organic carbon observed in cloud water. 69 In the box model, only the measured k OH values for dimers were used, and the rate coefficients estimated with the structure−activity relationship (SAR, Section S5) at 298 K (k OHd SAR ) were used for the rest of the TA under investigation. The k OHd SAR values listed in Table S2 were used for TAs because measured values for the lower-MW products were noticeably affected by their secondary formation from R(I). The yields of formation of the individual α-P SOAd aq were obtained by fitting the model to the experimental data.

Control Experiments and Uncertainty.
Each measurement was carried out a minimum of three times, and the uncertainties reported are propagated from the experimental uncertainties that were derived as 2σ values. Control experiments were carried out to verify if the decrease in the concentration of the TAs under investigation was only due to the reaction with the OH. When the photoreactor lamps were kept off, no reactions between α-P SOAd aq and H 2 O 2 were observed. Aliquots of the reaction solution were also stabilized by decomposing the leftover H 2 O 2 (Section 2.4) to ensure that no further reaction occurred in the LC/MS autosampler rack ( Figures S1 and S2). Ketoacids, including cis-pinonic acid, underwent slow photolysis induced by the 310 nm irradiation used in the photoreactor. 51 The first-order disappearance rates (photolysis and hydrolysis) derived from the control experiments (dark and UV-only) were subtracted from the overall decay rates to obtain the k OH values. 51

Acids and Oligomers Detected in α-P SOAd aq .
Structures for the detected α-P SOAd aq were proposed based on the acquired HR-MS and MS/MS spectra (Section S4) and 70−73 the literature data (Table 1). 10 15 Chromatographic peak areas were used to estimate the relative concentrations of individual TAs and dimer esters. α-P SOA was dominated by early-stage products, cis-pinonic, cispinic, terpenylic, diaterpenylic, hydroxy, and oxo-pinic acids, diaterpenylic acid acetate as well as dimers with MWs 338, 358, and 368 Da, with some contribution from the C 4 -ringopening compounds (Table 1), 29,31,57 that were likely formed from the reaction with the OH due to the absence of the scavenger in the flow tube.
At the same time, the formation of some markers of the gasphase OH aging of α-P SOA , including MBTCA and hydroxydicarboxylic acids, [21][22][23][24]75,83 was not observed (see also Figure S3). Formation of MBTCA from the gas-phase oxidation of cis-pinonic acid and pinonaldehyde by the OH was reported. 21,22,72,84,85 Furthermore, some studies tentatively identified MBTCA as one of the products of the aqueous OH oxidation of cis-pinonic 26,29,31 and cis-pinic acids. 33 However, MBTCA was not reported as the major product of the aqueous OH aging of α-P SOA , 49 which is consistent with the results acquired in this work. Here, the absence of some of the common α-P SOA OH aging markers may be due to their low formation yields in the aqueous solution or insufficient aging time since MBTCA was shown to be the higher-generation product of the OH oxidation of terpenoic precursors. 22,26,31 Also, the OH aging in the bulk solution used in this work may not be an entirely adequate representation of the cloud-water processing of terpenoic SOAs, 57 which can result in low formation yields for some of the aging products encountered in the ambient PM. There is a need to further investigate the formation of MBTCA and other typical gas-phase OH aging markers in the aqueous phase and at the air−water interface.

Kinetic Measurements.
The k OH values measured in this work were compared with the values predicted using the SAR (Section S5) at 298 K (Table 2) and with the literature data. 51,86,87 In solution when every encounter between reactants leads to a reaction, the reaction rate is diffusionlimited. 88 The k diff values predicted (Section 2.4) that the rates of the individual α-P SOAaq with the OH are below the diffusionlimited values; hence, they are only partially (up to 33%) controlled by diffusion (Table S2).
The k OH values measured in this work under acidic and basic conditions for cis-pinic, cis-pinonic, and 4-oxopinonic acids are generally in good agreement with the previously reported data (Table 2). However, the k OH values measured here for some TAs were lower than the values obtained using pure standards. 49,51 The estimated k SAR values (Table S5) were also noticeably higher than the k OH values measured in this work (Table 2). 49,51 Therefore, k OH values measured here for lower-MW products are likely affected by their secondary formation in the reaction solution. 29,[31][32][33]89 Previously, only the formation of terpenylic acid (m/z 171) was observed from reaction 1 under very similar conditions. 49 Note, however, that the authors did not compare their results with SAR predictions and used slightly outdated kinetic data; their study was focused on a lower number of TAs, 49 which might explain these discrepancies. Nevertheless, the kinetic data presented here  (Tables 2 and S5) show that the OH reactivity of TAs under investigation is within the range 10 8 −10 10 M −1 s −1 for the organics encountered in the atmosphere. 46

Aqueous OH Aging of α-P SOAaq .
To better understand the mechanism of aging upon reactions with OH, a box model was set up. Estimated k SAR values (Table S5), measured k OH , and literature data compiled in our previous study 51 all showed little pH dependence of OH reactivity of the TAs under investigation. Average k SAR values were used in the box model for lower-MW products because the measured values were affected by the formation of the TAs under investigation from R(I) ( Figure S15).
The concentrations of α-P SOAd aq increased during R(I) ( Table  2). Because 8-hydroxy-pinonic, cis-pinonic, and cis-pinic acids were the major components of α-P SOAd aq , they were selected as the precursors of the OH aging markers (Section 2.5). The precursors for the rest of the TAs under investigation were assigned based on the HR-MS measurements (Table 1) and the data available in the literature (Table 2). 29,31,33,89 A relatively good agreement between the experimental data and modeling results was obtained for the early-stage products of α-P oxidation ( Figure S16).
An initial increase of concentrations of C 4 -ring-opening products ( Table 1) was observed during R(I), including terpenylic acid and its derivatives, 33,49 which is accurately reproduced by the kinetic model ( Figure 1). Furthermore, to the best of our knowledge, this is the first observation of the formation of diaterpenylic acid and diaterpenylic acid acetate from R(I) in an aqueous solution. The modeled yields of higher-generation products from cis-pinonic, cis-pinic, and 8hydroxy-pinonic acids ranged from 1−2 to 25−35% with higher values corresponding to the formation of terpenylic acid ( Figure S18). The model yields of terpenylic acid are unrealistically high, which points out the relevance of the OH reaction with the non-acidic, terpenoic precursors; 74 similar conclusions can be presented about the modeled yields of cis-pinonic and cis-pinic acids from pinonaldehyde and norpinonaldehyde ( Figure S18).
Modeling results also revealed that a significant fraction of the products of R(I) is not represented in the model ( Figure  S18 and Table S6). Therefore, a large portion of α-P SOAd aq can be volatilized and concerted into lower-MW and 31,90 nonacidic products that are not quantified under the LC/MS analysis conditions used in this work. C 2 −C 4 acids are practically not retained by the C 18 column used, and their ionization efficiency in ESI is likely low. These results imply that a large fraction of alkoxy radicals (RO) formed from R(I) likely undergoes the β-scission reaction, yielding lower-MW products, 91 like oxalic acid, which is abundant in cloud water. 8 In the box model, oligomers are decomposed by the OH (Figure S17), 48,49 which is consistent with the acquired experimental data. At the same time, the formation of pinyldiaterpenyl ester (MW = 358 Da) was observed from the OH aging of α-P SOA under dry (RH < 1%) conditions; 75 a radicalmediated mechanism is most likely involved in the formation of this dimer ester. 18,75 Therefore, the formation of some acidic dimers from the reaction of TAs with the OH likely involves interfacial reactions of RO and peroxy (RO 2 ) radicals. 75,92 An increase in the concentration for some of the functionalized derivatives of cis-pinic and cis-pinonic acids, including compounds with MW 158 Da, was reported following gasphase OH aging of α-P SOA , 75 whereas in this work, the concentrations of C 4 -ring-retaining products decreased during R(I) (Figure S16). Such a result likely reflects the different composition of α-P SOA in the gas and aqueous phases. α-P SOAd aq is enriched in highly soluble TAs, whereas in the gas phase, higher amounts of more volatile carbonyls (aldehydes) were observed. 67,68 These carbonyls are likely precursors of the C 4ring-retaining TAs, resulting in an observed increase in their concentration following gas-phase OH aging of α-P SOA . 75,93 Analysis of specific dimers (this work), as well as previously reported results of non-targeted analyses, revealed that oligomers either are not formed or are decomposed following the aqueous OH reaction with terpenoic SOAs at cloudrelevant and also at higher concentrations of the precursors. 33,48,49,53 At the same time, the formation of higher-MW products was observed from the aqueous OH oxidation of C 2 − C 4 precursors carried out in the bulk solution. 61,94,95 However, in our previous work, increasing the concentration of cispinonic acid to 10 mM still did not result in the formation of higher-MW products during the aqueous oxidation by the OH. 29 Evidently, very high concentrations of the TAs are required for oligomer formation, which may occur in the atmosphere during evaporation of droplets. These conclusions are supported by the reported formation of oligomers in aerosols composed of saturated diacids. 92 Likely that at cloudrelevant concentrations used in this work, alkyl radicals will react primarily with molecular oxygen, suppressing the formation of higher-MW products in aqueous solutions. 61,95 The dimers detected here also underwent slow hydrolysis, under acidic and basic conditions, which was accompanied by a noticeable increase in the concentrations of terpenylic acid (Figures S1 and S2). However, within the time scale of the photooxidation experiments, hydrolysis was a minor process; therefore, dark hydrolysis of dimer esters is unlikely to compete with the OH-mediated reactions under the realistic atmospheric conditions. 92

ENVIRONMENTAL IMPLICATIONS
The atmospheric lifetimes of TAs under investigation due to the reaction with the OH were calculated with eq 2. 88 i k j j j y In eq 2, τ is the total lifetime for a given compound due to the reaction with the OH in both gas and aqueous phases, ω is the liquid water content (LWC) (m 3 /m 3 ), k OHd g and k OHd aq are the k OH values in the gas and the aqueous phase at 298 K, respectively, and H OH cc and H cc are the dimensionless Henry's law constants for the OH (764) 96 and TAs (Table S8), respectively.
[OH] aq is the cloud and fog water concentration of OH (M), 46,97 which is connected with the [OH] gas via Henry's law equilibrium (eq 2). 88 The k OHd g values were estimated with SAR, 98,99 and the H cc values were estimated with HenryWin 4.11 (Table S8). 100 The six TAs, identified as the major components of α-P SOAd aq (Table 1), will undergo oxidation by the OH primarily in the aqueous phase when the LWC is ≥1 × 10 −3 (g/m 3 ) ( Figure  3). 26,29,101 Furthermore, dimers are expected to react in the aqueous phase, even in aerosols and haze with the LWC as low as 1 × 10 −6 (g/m 3 ) ( Figure S19). These results indicate that in clouds with the LWC between 0.01 and 1 (g/m 3 ), the aqueous Environmental Science & Technology pubs.acs.org/est Article OH aging of α-P SOA will occur almost exclusively in the aqueous phase. At the same time, the OH aging of α-P SOA under dry conditions is more likely to yield new oligomers, which are classified as extremely low-volatility compounds 14 and are expected to exist exclusively in the particle phase. 19 Functionalization of TAs under investigation following R(I) is also unlikely to contribute to an increase in acidity via the formation of acids with lower pK a values (Table S10). 65,102,103 Instead, aging of SOAs may enhance the acidity of aqueous particles via the formation of lower-MW acids (e.g., oxalic and formic acids) from R(I). 9 The results obtained in this work showed a high degree of fragmentation of TAs and rapid decomposition of acidic dimers; oligomers present in SOAs are classified as extremely low-volatility compounds. 14,104 These results argue against an efficient formation of aq SOAs from R(I). Conversely, very high  Environmental Science & Technology pubs.acs.org/est Article mass yields of aq SOAs from the OH reaction with terpenoic precursors (40−60%) were reported. 26,26 As previously reported, evaporation of nebulized aqueous solutions of TAs did not result in the dark formation of aq SOAs. 26 It is therefore likely that functionalized, non-acidic products of R(I) undergo non-radical accretion reactions 105 during droplet evaporation. These dark reactions can contribute to the formation of aq SOAs from terpenoic precursors, like oligomer formation from carbonyls, 106 that are formed from RO 2 radicals via Russell or Bennett−Summers mechanisms. 107 The reported yields of TAs from the α-P + O 3 reaction are as high as 40%; 108 therefore, the formation of terpenoic aq SOAs from R(I) is likely to occur in the atmosphere. The lifetimes estimated for the early-stage products ( Figure  2) and dimers ( Figure S19) corresponded to the in-cloud processing time between 2.5 weeks and 2 min, depending on the cloud-water concentration of OH. 64 These results strongly indicate that R(I) can efficiently compete with other removal mechanisms, like photolysis or wet deposition, 27 even after taking into account the fact that clouds are present about 15% of the time. 26,49 Very recently, a large, light-driven formation of OH was reported in cloud droplets, resulting in the [OH] aq as high as 3.5 μM, 109 which is six orders of magnitude higher as compared with the upper limit [OH] aq in marine clouds ( Figure 2). 64 Following this burst of OH aq , the gas-phase OH aging of α-P SOA may become practically irrelevant in clouds, thereby further enhancing the cloud-water formation of terpenoic aq SOAs.
First-generation products and dimers are removed following aging by the OH, which results in about a 10% increase in the average O/C ratio of TAs contributing to α-P SOAd aq . Moreover, a 3-fold decrease in the overall OH reactivity of α-P SOAd aq was observed (Figure 3), which can be attributed to the formation of more oxidized, lower-MW products ( Figure S14), which significantly lowers the OH-scavenging ability of the aged aq SOA. The results presented in Figure 3 also underline the possibility of a simplified representation of the in-cloud formation of aq SOAs from R(I). 44,49,110 The concentrations of the C 4 -ring-opening products, including newly identified OH aging markers as well as terpenylic acid and related compounds, increased several-fold during R(I); 26,31,32,57 these compounds, however, are not exclusively formed in the aqueous phase. 75 The kinetic data acquired in this work indicate that saturated oligomers are more reactive toward OH as compared with the majority of the lower-MW products (Figures S14 and S15).
List of materials and reagents, results of control experiments, results of HR-MS measurements and MS/MS analysis conditions, derivations of k diff and k SAR values and comparison with the experimental data, MS 2 spectra and proposed fragmentation pathway for some α-P SOAd aq , details of the kinetic box model, k OH (gas and aqueous) values and H cc values for all α-P SOAd aq , and measured and estimated pK a values for all TAs investigated (PDF) ■

■ ACKNOWLEDGMENTS
This project was funded by the Polish National Science Centre: grant number UMO-2018/31/B/ST10/01865. We thank Professor Aleksandra Misicka-Kęsik for making the LC/ MS measurements possible. This study was carried out at the Biological and Chemical Research Centre, University of Warsaw, established within the project co-financed by the European Union from the European Regional Development Fund under the Operational Programme Innovative Economy, 2007−2013. We also thank the anonymous reviewers for their insightful comments and suggestions that helped to enhance the scientific quality of this article. ■ REFERENCES